Fermentors are the closed vessels which are used for production of product’s(cell mass and metabolites) on large scale.
The most common among these is the stirred tank fermentor where the impellers are fitted to stir the medium Stirring of medium is done forcibly.

Working Principle
Microbial growth will with simultaneous decline in product production. Hence forced aeration favors rapid growth of micro-organisms. Secondly, pH of the culture medium also declines sharply with growing microbial cells. Therefore, pH probe fitted with the fermentor regularly monitors the pH and maintains at optimum by adding avid or alkali.